1. Field of the Invention
The present invention relates to a read head spin valve sensor that has a triple antiparallel (AP) coupled free layer structure and more particularly to such a free layer structure which has first and second cobalt based layers for increasing a magnetoresistive coefficient of the read head and a nickel iron based layer between the cobalt based layers for increasing responsiveness of the read head to signal fields.
2. Description of the Related Art
A spin valve sensor is employed by a read head for sensing magnetic fields on a moving magnetic medium, such as a rotating magnetic disk. A typical sensor includes a nonmagnetic electrically conductive first spacer layer sandwiched between a ferromagnetic pinned layer structure and a ferromagnetic free layer structure. An antiferromagnetic pinning layer interfaces and is exchange coupled to the pinned layer structure for pinning a magnetic moment of the pinned layer structure 90.degree. to an air bearing surface (ABS) where the ABS is an exposed surface of the sensor that faces the rotating disk. First and second leads are connected to the spin valve sensor for conducting a sense current therethrough. A magnetic moment of the free layer structure is typically oriented parallel to the ABS in a quiescent condition, the quiescent condition being where the sense current is conducted through the sensor in the absence of any signal fields. The magnetic moment of the free layer structure is free to rotate from the parallel position in response to signal fields from the rotating magnetic disk.
The thickness of the spacer layer is chosen so that shunting of the sense current and a magnetic coupling between the free and pinned layer structures are minimized. This thickness is typically less than the mean free path of electrons conducted through the sensor. With this arrangement, a portion of the conduction electrons are scattered at the interfaces of the spacer layer with respect to the pinned layer structure and the free layer structure. When the magnetic moments of the pinned and free layer structures are parallel with respect to one another scattering is minimal and when their magnetic moments are antiparallel scattering is maximized. Changes in scattering, in response to field signals from the rotating disk, changes the resistance of the spin valve sensor as a function of cos .theta., where .theta. is the angle between the magnetic moments of the pinned and free layer structures. The sensitivity of the sensor is quantified as magnetoresistive coefficient dr/R where dr is the change in resistance of the sensor between parallel and antiparallel orientations of the pinned and free layer structures and R is the resistance of the sensor when the moments are parallel.
The pinned layer structure may be a simple pinned layer structure or an antiparallel (AP) pinned layer structure. The simple pinned layer structure is a single ferromagnetic layer which may comprise one or more ferromagnetic films. The AP pinned layer structure comprises a nonmagnetic antiparallel (AP) coupling layer located between and interfacing ferromagnetic first and second antiparallel (AP) pinned layers. In the AP pinned layer structure one of the AP pinned layers is pinned by the pinning layer and the other AP pinned layer is strongly antiparallel coupled to the pinned AP pinned layer through the AP coupling layer. Accordingly, the magnetic moments of the first and second AP pinned layers are antiparallel with respect to each other. The AP pinned layer structure is fully described in commonly assigned U.S. Pat. No. 5,465,185 which is incorporated by reference herein.
A spin valve sensor is also classified as a single spin valve sensor or a dual spin valve sensor. In a single spin valve sensor only one pinned layer structure and one pinning layer are employed wherein the pinned layer structure is separated from the free layer structure by only one spacer layer. A dual spin valve sensor employs a ferromagnetic free layer structure which is located between first and second ferromagnetic pinned layer structures wherein a first spacer layer separates the first pinned layer structure from the free layer structure and a second spacer layer separates the second pinned layer structure from the free layer structure. The first and second pinned layer structures are pinned by first and second antiferromagnetic pinning layers. In comparison to the single spin valve sensor the magnetoresistive coefficient of the dual spin valve sensor is increased by a factor of approximately 1.4 due to the spin valve effect on each side of the free layer structure.
In either the single or dual spin valve sensor the magnetic spins of the single or dual pinning layer structures must be set so as appropriately pin the magnetic moment of the respective pinned layer structure. This is accomplished by raising the temperature of the sensor at or above the blocking temperature of the pinning or pinning layers in the presence of a field that is oriented perpendicular to the ABS. The field orients the magnetic moment of the one or more pinned layer structures perpendicular to the ABS. The blocking temperature is the temperature at which all of the magnetic spins of the pinning layer align with the orientation of the magnetic moment of the pinned layer structures at their interface. When the sensor cools the magnetic spins of the pinning layer are set perpendicular to the ABS and pin the magnetic moment of the pinned layer structure perpendicular to the ABS. After fabrication and installation in a disk drive a portion or all of the magnetic spins of a pinning layer may become disoriented due to imposition of a magnetic field in the presence of heat from the drive or from an electrostatic discharge (ESD). Upon this occurrence it is important that the sensor permit the magnetic spins to be reset by employing a current pulse through the sense current circuit that will produce the required sense current fields in the presence of heat for resetting the one or more pinning layer structures.
A transfer curve (readback signal of the spin valve head versus applied signal from the magnetic disk) of both the single or dual spin valve sensor is a substantially linear portion of the aforementioned function of cos .theta.. The greater this angle, the greater the resistance of the spin valve to the sense current and the greater the readback signal (voltage sensed by processing circuitry). With positive and negative signal fields from a rotating magnetic disk (assumed to be equal in magnitude), it is important that positive and negative changes of the resistance of the spin valve sensor be equal in order that the positive and negative magnitudes of the readback signals are equal. When this occurs a bias point on the transfer curve is considered to be zero and is located midway between the maximum positive and negative readback signals. When the direction of the magnetic moment of the free layer is parallel to the ABS in the quiescent state the bias point is located at zero and the positive and negative readback signals are equal when sensing positive and negative signal fields from the magnetic disk. The readback signals are then referred to in the art as having symmetry about the zero bias point. When the readback signals are not equal the readback signals are asymmetric which equates to reduced storage capacity of a magnetic disk drive.
The location of the bias point on the transfer curve is influenced by three major forces on the free layer. In a single spin valve sensor these forces are a demagnetization field (H.sub.D) from the pinned layer structure, a ferromagnetic coupling field (H.sub.F) between the pinned layer structure and the free layer structure, and sense current fields (H.sub.l) from all conductive layers of the spin valve except the free layer. When the sense current is conducted through the spin valve sensor, the pinning layer (if conductive), the pinned layer structure and the first spacer layer, which are all on one side of the free layer structure, impose sense current fields on the free layer structure that rotate the magnetic moment of the free layer in a first direction. The ferromagnetic coupling field from the pinned layer further rotates the magnetic moment of the free layer in the first direction. The demagnetization field from the pinned layer on the free layer rotates the magnetic moment of the free layer in a second direction opposite to the first direction. Accordingly, the demagnetization field opposes the sense current and ferromagnetic coupling fields and can be used for counterbalancing. In a dual spin valve sensor each of the pinned layer structures exerts a demagnetization field (H.sub.D) and a ferromagnetic coupling field (H.sub.F) on the free layer structure and there are additional sense current fields exerted on the free layer structure. The mode of balancing these fields in order to establish a zero bias point for the free layer structure is now different than the single spin valve sensor and needs to be carefully analyzed in order to obtain the required balance and enable reset if the magnetic spins of the pinning layers become disoriented.
Over the years a significant amount of research has been conducted to improve the magnetoresistive coefficient dr/R of spin valve sensors without adversely affecting other performance factors such as biasing of the free layer and thermal stability of the pinning layers. These efforts have increased the storage capacity of computers from kilobytes to megabytes to gigabytes.